This is the key: the Novation Impulse 49 does not carry enough power down the USB pipeline. I had the same problem for months - hit a few keys on my keyboard which would play the midi notes on Logix X software instruments - after a while it would just cut out.
The solution i found worked was - i just bought a cheap independant USB powered hub. Like this (bellow) works perfectly fine now. can press all keys on my novation keyboard and all midi notes deliver to Logix X and it never cuts out.
I'm using Logic X running on Yosemite with a New Mac Pro trashcan. Also works with my Mac 4.1